When your backup job failed with an error message, "Unable to connect to resource .....", you should expand the job log by clicking on the Expand All button and examining each of the resource that was backed up. Do not rely on the resource given in the "unable to connect" message.
My BE media server is also my Lotus Domino server. A couple of weeks ago, my job failed with "Unable to connect to resource C: drive". This message makes no sense because the C: drive on my media server can definitely be accessed. Otherwise, BE will not be able to run. I ran a test to backup the C: drive and it did not fail, but the following night's backup again failed when it is unable to connect to the C: drive. It took a couple of failure before I expanded the joblogs and saw that the C: drive backed up fine, but the Lotus Domino databases were not being backed up. I realised that the jobs were unable to connect to the Lotus Domino databases on the C: drive, not the C: drive itself. Somehow, the interface between BE and Domino died. I restarted the BE services and everything is fine from then on.
Hence, to trouble-shoot this kind of problems, you got to expand your joblog and look at each resource that is being backed up.